In high pressure discharge lamps generally used at present, for example, headlights for automobiles and backlights for projectors, since rupture may be caused during operation at high temperature and high pressure when the thickness distribution of the discharge bulb is not uniform, discharge bulbs of uniform wall thickness with no exhaust tube remains (tips) in the light emitting portion are used and they are also referred to as tipless lamps.
A tipless lamp of this type is manufactured by a method of using, for example, as shown in FIG. 5(a) to (c), a quartz bulb 51 having a chamber 53 formed as a light emitting portion to a longitudinally intermediate portion of a quartz tube and straight tubular sealing portions 52A and 52B formed on both ends thereof, and electrode mounts 57 each formed by welding a tungsten electrode 56 by way of a molybdenum foil 55 to the top end of a lead wire 54, inserting the electrode mounts 57 inside the quartz bulb 51, and sealing them in a state of keeping the inside at a negative pressure.    [Patent Document 1] JP No. 3653195
In this case, for conforming the arc length (inter-electrode distance) to a designed value, it is required to position the top end of the electrode 56 at a predetermined position in the chamber 53.
Then, in the existent electrode mount 57, a pressing portion 58 for tube inner wall is formed by bending a lead wire 54 into a W-shaped configuration.
With the constitution described above, when the electrode mount 57 is inserted into the quartz bulb 51, since the pressing portion 58 is pressed to the inner peripheral surface of the quartz bulb 51 to result in frictional force, and, as a result, the electrode mount 57 can be fixed temporarily at an optional portion, the electrode mount 57 can be positioned without displacement unless external force is exerted.
In the same manner, in Patent Document 2, a U-shaped pressing portion for tube inner wall is formed to an electrode mount such that the electrode mount can be fixed temporarily with no positional displacement.    [Patent Document 2] JP-A-2000-21312
However, it is difficult by the method of causing the frictional force and fixing the electrode mount 57 at an optional position, it is difficult to confirm whether the electrode mount is positioned to an accurate position or not. Then, the electrode mount is inserted to an appropriate position while monitoring the position by using a magnifying scope such as a CCD camera, this undergoes a lens effect such as deflection or distortion of glass, tends to cause errors in adjustment and result in scattering of an arc length.
Further, upon sealing, after temporarily sealing the upper end 59, sealing is conducted from the vicinity of the tungsten electrode 56 to the molybdenum foil 55 and the lead wire 54 successively (refer to FIG. 5(b)).
In this case, since not only the electrode mount 57 is positioned by the pressing portion 58 for a tube inner wall in the W-shaped or U-shaped configuration but also the upper end 59 is temporarily sealed, the molybdenum foil 55 tends to be twisted due to thermal expansion of the molybdenum foil 55 and such twisting of the molybdenum foil 55 may lead to the leakage in the sealing portion which may possibly cause failure in the manufacture.
Therefore, it has been proposed a method of manufacturing a lamp of providing a protrusion 63 to the inner circumference of a portion as sealing portions 62 and 62 of a quartz bulb 61 (refer to FIG. 6(a)) or forming an inner flange 65 between a light emitting portion 64 and a sealing portion 62 of a quartz bulb 61 as shown in FIG. 6(b), and engaging the positioning engagement portion 66 to the protrusion 63 or the inner flange 65 using an electrode mount 67 having a positioning engagement portion 66 formed at a position spaced apart by a predetermined length from the top end of the electrode, thereby capable of accurately positioning the top end of the electrode.    [Patent Document 3] JP-A-H06-290748
However, in a case of forming the protrusion 63 to the inner surface of the sealing portion 62, it is necessary to inwardly deform a portion where the protrusion is to be formed by pressing a roller 68 to the portion while heating the portion. Such fabrication is not only troublesome but also may vary the shape of the protrusion 63 depending on the size of the quartz tube (particularly, for wall thickness) or other heating condition, etc. and the arc length tends to vary in a case where the electrode is positioned with the protrusion 63 as a reference.
Further, in a case of forming the inner flange 65 between the light emitting portion 64 and the sealing portion 62, since it is necessary to insert a router 69 from the opening of the quartz bulb 61 and ream the inner surface of the sealing portion 62 made of a quartz tube of a high hardness, it involves a problem that the fabrication is difficult tending to cause cracking, the yield is low, and the productivity is poor.
Further, for removing thermal strains in the tipless lamp during fabrication, annealing is generally conducted after completing the lamp and higher pressure resistant strength can be obtained by applying annealing at an appropriate temperature.    Patent Document 4: JP-A-2004-335457
However, since the temperature during annealing is extremely high, metal leads exposed to the outside are oxidized when the completed lamp is annealed. Accordingly, for annealing, a special device such as a vacuum baking furnace, a vacuum heating furnace, etc. capable of performing heat treatment in a vacuum atmosphere are necessary.
Further, for satisfying the requirement for higher efficiency and longer working life of the lamp, quartz at high purity has been used for the quartz bulb and the material cost is outstandingly expensive compared with existent quartz. When a tipless lamp is manufactured, since both ends are cut off, the quartz bulb has to be formed previously to a sufficient length more than that of an actual lamp and, since expensive quartz tube at an extremely high purity has to be used also for a portion which is finally cut off and discarded, there is a problem that the material cost is increased.